1. Field of the Invention
This invention relates to precursors for prostaglandin analogues and processes for preparing the same. More particularly, this invention relates to bicyclolactone compounds and .gamma.-lactone sulfide and sulfoxide compounds represented by the formulae (I) and (II), respectively, hereinafter described which are useful as precursors for the synthesis of prostaglandin analogues, that is, 10-oxaprostaglandins, having biological properties similar to prostaglandin compounds, and processes for prepring such precursors for prostaglandin analogues.
2. Description of the Prior Art
It is well known that the naturally-occurring prostaglandin compounds are composed of 20 carbon atoms and contain in the structure thereof a cyclopentanone ring and exist broadly in the brain, lung, kidney, semen, uterus membrane, etc. of living body. These prostaglandin compounds are also known to have a wide variety of excellent pharmacological activities such as anti-ulcer, hypotensive, anti-asthmatic, uterotonic activities depending upon critical differences in the chemical structure of prostaglandin compounds, and recently the synthesis of prostaglandin compounds has been extensively studied.
Further, it has been found that the prostaglandin compounds having a hetero atom, e.g., an oxygen, nitrogen or sulfur atom, as a ring member of the cyclopentane ring also possess activities similar to those of naturally-occurring prostaglandin, as reported in Taisha (Metabolism), Vol. 21, 1461 (1975). In particular, prostanoic acid as a basic structure of prostaglandin compounds in which the carbon atom at the 10-position is replaced by an oxygen atom is called as 10-oxaprostanoic acid and various methods for the synthesis of 10-oxaprostanoic acid have been reported. For example, a lactone ring as a basic structure can be produced by oxidation of a cyclooctene compound, as described in F. M. Hauser and R. C. Hoffman, Tetrahedron Letters, 905 (1974), or by condensation of a succinic acid ester with formaldehyde, as described in Japanese Patent Application Laid Open to Public Inspection Nos. 8773/1973 and 8774/1973.
The above conventional methods for the formation of a lactone ring possess certain characteristic features, but are not advantageous in that these methods require expensive reagents for the carbon-carbon extension reaction and are unable to produce selectively specific stereoisomers required for producing pharmacologically active prostaglandin compounds. Particularly, in the above conventional methods, it is difficult to produce stereospecifically a compound having an .alpha.-configuration of hydroxy group at the 15-position which is required for the structure of naturally-occurring prostaglandin.